1. Field of the Invention
The invention relates to an automatic transmission, and more particularly to an automatic transmission capable of providing seven forward gear stages or speeds by using three planetary gear sets and also capable of shifting from one gear stage to another gear stage by changing a combination of two different engaging elements, such as clutches and brakes, to be engaged.
2. Description of Related Art
Automatic transmissions of motor vehicles, each including a plurality of planetary gear sets, clutches and brakes, have been widely used. Japanese Laid-Open Patent Publication No. 2000-266138 (JP-A-2000-266138) discloses an example of such an automatic transmission which is able to provide seven forward gear stages by using four planetary gear sets. Also, Japanese Laid-Open Patent Publication No. 2001-82555 (JP-A-2001-82555) discloses another example of the automatic transmission which is able to provide ten forward gear stages by using three planetary gear sets.
The automatic transmission as disclosed in JP-A-2000-266138, which uses four planetary gear sets, has a relatively large axial length, and may suffer from a difficulty in installing the transmission on the vehicle, an increased weight, and an increased manufacturing cost. On the other hand, the automatic transmission as disclosed in JP-A-2001-82555 can be comparatively easily installed on the vehicle. However, there is a need to change a combination of the maximum of four engaging elements (clutches and/or brakes) to be engaged so as to shift the transmission from one speed to another speed. Thus, the automatic transmission requires complicated, high-accuracy shift control, and may suffer from shift shocks.